This invention relates to probes for use in measurement on positioning machines, such as machine tools.
It is known to provide a probe on a machine tool. The probe may for example be mounted in the tool-holding spindle of the machine tool, in order to measure a workpiece placed on the bed or table of the machine tool. Because such a probe is interchangeable with cutting tools, it is not possible to provide permanent wiring to transmit its signal back to the machine""s controller. Consequently, it is common to provide a wireless signal transmission system, particularly an optical (e.g. infrared) transmission system. An optical transmitter such as one or more light emitting diodes is provided on the probe housing, and sends signals to an optical receiver positioned at a convenient place on the fixed structure of the machine.
Such probes need to be battery powered. To conserve battery life, it is desirable that they should only be switched on when required for use. It is known, e.g. from European Patent No. 337669 to provide an optical switch-on or start signal. In a product commercially available from Renishaw plc, an optical transmitter is provided in the same module as the optical receiver mounted on the fixed structure of the machine. When the probe is to be switched on, this transmits a burst of optical pulses to an optical receiver mounted on the housing of the probe. The probe includes a detector circuit, drawing very low quiescent current, which detects the burst of pulses and switches on the rest of the probe circuitry.
Such a switch-on or start signal system can operate incorrectly, as a result of optical interference. Such interference can be generated for example, by xenon beacons on adjacent machine tools, and by fluorescent lighting systems using a switched mode power factor correcting ballast. In recent years, these have been used increasingly, and so the problem of interference is becoming more significant. When a probe is switched on incorrectly, its battery life is reduced. In addition, in a few cases more extreme problems have been noted, where the signal from an incorrectly switched on probe has been received by the machine""s controller, causing it to take unwanted action.
The commercially available probes mentioned above include a band pass filter tuned to the same frequency as the pulses of the switch-on signal, in order to exclude interference. However, it has been found that some interference problems still remain.
The present invention results from research carried out by the inventors to analyse the nature of the interference. They have found that interference sources such as xenon beacons and power corrected fluorescent lighting can produce wide band interference, which is not completely removed by the band pass filter.
The inventors also discovered that the interference has characteristics different from the burst of pulses used as a start signal, and the present invention uses some of those differences to differentiate between interference and a genuine start signal.
Accordingly, the present invention provides a probe having a wireless signal transmission circuit, a detecting circuit for detecting an optical switch-on signal and switching on the signal transmission circuit when the switch-on signal is received, the detecting circuit including a time discriminating circuit which discriminates whether an input signal is a switch-on signal or interference on the basis of the duration of the switch-on signal.